1. Field of the Invention
The present invention relates to a photopolymerizable composition and a photopolymerizable lithographic printing plate. Particularly, it relates to a photopolymerizable composition showing high sensitivity to a light in a long wavelength region, a photopolymerizable lithographic printing plate and a process for forming an image.
2. Discussion of Background
Heretofore, as methods for forming images by exposure of photopolymerizable compositions, various methods have been known, such as, a method wherein a layer of a photopolymerizable composition comprising an ethylenically unsaturated compound and a photopolymerization initiator, or comprising an organic polymer binder material in addition to them, is formed on the surface of a support and then subjected to image exposure so that the ethylenically unsaturated compound at the exposed portion is polymerized and cured, and then the non-exposed portion is dissolved and removed to form a cured relief image, a method wherein a layer of a photopolymerizable composition is exposed to change the bond strength of the layer to the support, and then the support is peeled to form an image, and a method wherein a change in the adhesion of a toner to the layer of a photopolymerizable composition due to light, is utilized to form an image. As the photopolymerization initiator, one which is sensitive to light with a short wavelength within an ultraviolet region of at most 400 nm, has been used, such as benzoin, benzoin alkyl ether, benzyl ketal, benzophenone, anthraquinone, benzyl ketone or Michler's ketone.
On the other hand, in recent years, a photosensitive material showing high sensitivity to a light within a visible light range has been strongly demanded along with the development of the image-forming technology, and many photopolymerizable compositions have been proposed, whereby the sensitivity range has been broadened to about 500 nm corresponding to a laser printing plate making system employing an oscillation beam of an argon ion laser of 488 rm. Further, researches have been actively carried out on photopolymerizable compositions sensitive to a light within a long wavelength region exceeding 600 nm corresponding to a laser printing plate making system employing a He--Ne laser or a semiconductor laser or a copying technique for full color image.
For example, JP-A-58-29803 and JP-A-4-31863 disclose a photopolymerizable composition comprising an ethylenically unsaturated compound and a photopolymerization initiating system, wherein the photopolymerization initiating system comprises a cyanine dye having heterocyclic rings connected by a mono-, tri-, penta- or hepta-methine chain and having a certain specific structure, and a s-triazine derivative having a certain specific structure. Further, JP-A-4-106548 (U.S. Pat. No. 5,219,709) discloses a photopolymerizable composition, of which the polymerization initiating system comprises a squalirium compound having a certain specific structure and a s-triazine compound having a certain specific structure.
However, it is commonly known that with respect to the active radical-forming ability of a photopolymerization initiator, the sensitivity to a light with a wavelength exceeding 500 nm, particularly 600 nm, sharply decreases along with a decrease in the light excitation energy. To a light within such a long wavelength region, none of the conventional photopolymerizable compositions has been satisfactory from the viewpoint of the sensitivity, and in addition, there has been a problem that a photopolymerization reaction is likely to proceed during handling under a white fluorescent lamp, whereby it has been difficult to obtain a product of a uniform quality.